A reader asks:

What does spectrum mean when talking about light therapy? Is “sunlight” a type of spectrum for the lamp or just like sunlight outdoors. Does the spectrum matter? Thanks for your advice.


Any light source (the sun, lamps, candles, even fireflies!) produces visible energy of different wavelengths. When we use a narrow-spectrum lamp that produces a very narrow range of wavelengths, we see a pure color (red, green, blue, yellow, etc.). When a lamp mixes wavelengths across the visible spectrum, our sensation of color is reduced–we may see white, or pinkish white, or bluish white, etc. The sun, as well as incandescent lamps and fluorescent lamps, all mix wavelengths in different ways. It is incorrect to call any lamp a “sunlight lamp”, because no lamp matches the sun’s wavelength spectrum. Any manufacturer who claims to provide a sunlight lamp is deceiving customers, and should not be trusted. For light therapy, we recommend fluorescent light boxes that present a broad spectrum of wavelengths, yielding whitish light at high intensity (10,000 lux) while minimizing aversive visual glare. In order to do this, short visible wavelengths in the blue range need to be present, but they should not be emphasized for several reasons: they produce aversive glare, they can interact with medications to be harmful to the eyes, and over years of exposure they can directly harm the eyes and contribute to reduced vision in old age. In our judgment, the best spectral balance for light therapy is provided by white-light lamps of 3000-5000 Kelvin color temperature, not higher or lower. We recommend not using colored lights or “full spectrum” white light above 5000 Kelvin. For further information, see our store at www.cet.org/cet-online-store/.